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  <div class="section" id="module-asyncore">
<h1>18.6. <tt class="xref docutils literal"><span class="pre">asyncore</span></tt> &#8212; Asynchronous socket handler<a class="headerlink" href="#module-asyncore" title="Permalink to this headline">¶</a></h1>
<p>This module provides the basic infrastructure for writing asynchronous  socket
service clients and servers.</p>
<p>There are only two ways to have a program on a single processor do  &#8220;more than
one thing at a time.&#8221; Multi-threaded programming is the  simplest and most
popular way to do it, but there is another very different technique, that lets
you have nearly all the advantages of  multi-threading, without actually using
multiple threads.  It&#8217;s really  only practical if your program is largely I/O
bound.  If your program is processor bound, then pre-emptive scheduled threads
are probably what you really need.  Network servers are rarely processor
bound, however.</p>
<p>If your operating system supports the <tt class="xref docutils literal"><span class="pre">select</span></tt> system call in its I/O
library (and nearly all do), then you can use it to juggle multiple
communication channels at once; doing other work while your I/O is taking
place in the &#8220;background.&#8221;  Although this strategy can seem strange and
complex, especially at first, it is in many ways easier to understand and
control than multi-threaded programming.  The <tt class="xref docutils literal"><span class="pre">asyncore</span></tt> module solves
many of the difficult problems for you, making the task of building
sophisticated high-performance network servers and clients a snap.  For
&#8220;conversational&#8221; applications and protocols the companion <a title="Support for asynchronous command/response protocols." class="reference external" href="asynchat.html#module-asynchat"><tt class="xref docutils literal"><span class="pre">asynchat</span></tt></a>
module is invaluable.</p>
<p>The basic idea behind both modules is to create one or more network
<em>channels</em>, instances of class <a title="asyncore.dispatcher" class="reference internal" href="#asyncore.dispatcher"><tt class="xref docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a> and
<a title="asynchat.async_chat" class="reference external" href="asynchat.html#asynchat.async_chat"><tt class="xref docutils literal"><span class="pre">asynchat.async_chat</span></tt></a>.  Creating the channels adds them to a global
map, used by the <a title="asyncore.loop" class="reference internal" href="#asyncore.loop"><tt class="xref docutils literal"><span class="pre">loop()</span></tt></a> function if you do not provide it with your own
<em>map</em>.</p>
<p>Once the initial channel(s) is(are) created, calling the <a title="asyncore.loop" class="reference internal" href="#asyncore.loop"><tt class="xref docutils literal"><span class="pre">loop()</span></tt></a> function
activates channel service, which continues until the last channel (including
any that have been added to the map during asynchronous service) is closed.</p>
<dl class="function">
<dt id="asyncore.loop">
<tt class="descclassname">asyncore.</tt><tt class="descname">loop</tt><big>(</big><span class="optional">[</span><em>timeout</em><span class="optional">[</span>, <em>use_poll</em><span class="optional">[</span>, <em>map</em><span class="optional">[</span>, <em>count</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#asyncore.loop" title="Permalink to this definition">¶</a></dt>
<dd><p>Enter a polling loop that terminates after count passes or all open
channels have been closed.  All arguments are optional.  The <em>count</em>
parameter defaults to None, resulting in the loop terminating only when all
channels have been closed.  The <em>timeout</em> argument sets the timeout
parameter for the appropriate <tt class="xref docutils literal"><span class="pre">select()</span></tt> or <tt class="xref docutils literal"><span class="pre">poll()</span></tt> call, measured
in seconds; the default is 30 seconds.  The <em>use_poll</em> parameter, if true,
indicates that <tt class="xref docutils literal"><span class="pre">poll()</span></tt> should be used in preference to <tt class="xref docutils literal"><span class="pre">select()</span></tt>
(the default is <tt class="xref docutils literal"><span class="pre">False</span></tt>).</p>
<p>The <em>map</em> parameter is a dictionary whose items are the channels to watch.
As channels are closed they are deleted from their map.  If <em>map</em> is
omitted, a global map is used. Channels (instances of
<a title="asyncore.dispatcher" class="reference internal" href="#asyncore.dispatcher"><tt class="xref docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a>, <a title="asynchat.async_chat" class="reference external" href="asynchat.html#asynchat.async_chat"><tt class="xref docutils literal"><span class="pre">asynchat.async_chat</span></tt></a> and subclasses
thereof) can freely be mixed in the map.</p>
</dd></dl>

<dl class="class">
<dt id="asyncore.dispatcher">
<em class="property">
class </em><tt class="descclassname">asyncore.</tt><tt class="descname">dispatcher</tt><a class="headerlink" href="#asyncore.dispatcher" title="Permalink to this definition">¶</a></dt>
<dd><p>The <a title="asyncore.dispatcher" class="reference internal" href="#asyncore.dispatcher"><tt class="xref docutils literal"><span class="pre">dispatcher</span></tt></a> class is a thin wrapper around a low-level socket
object. To make it more useful, it has a few methods for event-handling
which are called from the asynchronous loop.   Otherwise, it can be treated
as a normal non-blocking socket object.</p>
<p>The firing of low-level events at certain times or in certain connection
states tells the asynchronous loop that certain higher-level events have
taken place.  For example, if we have asked for a socket to connect to
another host, we know that the connection has been made when the socket
becomes writable for the first time (at this point you know that you may
write to it with the expectation of success).  The implied higher-level
events are:</p>
<table border="1" class="docutils">
<colgroup>
<col width="35%" />
<col width="65%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Event</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal"><span class="pre">handle_connect()</span></tt></td>
<td>Implied by the first write event</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">handle_close()</span></tt></td>
<td>Implied by a read event with no data
available</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">handle_accept()</span></tt></td>
<td>Implied by a read event on a listening
socket</td>
</tr>
</tbody>
</table>
<p>During asynchronous processing, each mapped channel&#8217;s <a title="asyncore.dispatcher.readable" class="reference internal" href="#asyncore.dispatcher.readable"><tt class="xref docutils literal"><span class="pre">readable()</span></tt></a> and
<a title="asyncore.dispatcher.writable" class="reference internal" href="#asyncore.dispatcher.writable"><tt class="xref docutils literal"><span class="pre">writable()</span></tt></a> methods are used to determine whether the channel&#8217;s socket
should be added to the list of channels <tt class="xref docutils literal"><span class="pre">select</span></tt>ed or
<tt class="xref docutils literal"><span class="pre">poll</span></tt>ed for read and write events.</p>
<p>Thus, the set of channel events is larger than the basic socket events.  The
full set of methods that can be overridden in your subclass follows:</p>
<dl class="method">
<dt id="asyncore.dispatcher.handle_read">
<tt class="descname">handle_read</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_read" title="Permalink to this definition">¶</a></dt>
<dd>Called when the asynchronous loop detects that a <tt class="xref docutils literal"><span class="pre">read()</span></tt> call on the
channel&#8217;s socket will succeed.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.handle_write">
<tt class="descname">handle_write</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_write" title="Permalink to this definition">¶</a></dt>
<dd><p>Called when the asynchronous loop detects that a writable socket can be
written.  Often this method will implement the necessary buffering for
performance.  For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">handle_write</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
    <span class="n">sent</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">buffer</span><span class="p">)</span>
    <span class="bp">self</span><span class="o">.</span><span class="n">buffer</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">buffer</span><span class="p">[</span><span class="n">sent</span><span class="p">:]</span>
</pre></div>
</div>
</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.handle_expt">
<tt class="descname">handle_expt</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_expt" title="Permalink to this definition">¶</a></dt>
<dd>Called when there is out of band (OOB) data for a socket connection.  This
will almost never happen, as OOB is tenuously supported and rarely used.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.handle_connect">
<tt class="descname">handle_connect</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_connect" title="Permalink to this definition">¶</a></dt>
<dd>Called when the active opener&#8217;s socket actually makes a connection.  Might
send a &#8220;welcome&#8221; banner, or initiate a protocol negotiation with the
remote endpoint, for example.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.handle_close">
<tt class="descname">handle_close</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_close" title="Permalink to this definition">¶</a></dt>
<dd>Called when the socket is closed.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.handle_error">
<tt class="descname">handle_error</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_error" title="Permalink to this definition">¶</a></dt>
<dd>Called when an exception is raised and not otherwise handled.  The default
version prints a condensed traceback.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.handle_accept">
<tt class="descname">handle_accept</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.handle_accept" title="Permalink to this definition">¶</a></dt>
<dd>Called on listening channels (passive openers) when a connection can be
established with a new remote endpoint that has issued a <a title="asyncore.dispatcher.connect" class="reference internal" href="#asyncore.dispatcher.connect"><tt class="xref docutils literal"><span class="pre">connect()</span></tt></a>
call for the local endpoint.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.readable">
<tt class="descname">readable</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.readable" title="Permalink to this definition">¶</a></dt>
<dd>Called each time around the asynchronous loop to determine whether a
channel&#8217;s socket should be added to the list on which read events can
occur.  The default method simply returns <tt class="xref docutils literal"><span class="pre">True</span></tt>, indicating that by
default, all channels will be interested in read events.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.writable">
<tt class="descname">writable</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.writable" title="Permalink to this definition">¶</a></dt>
<dd>Called each time around the asynchronous loop to determine whether a
channel&#8217;s socket should be added to the list on which write events can
occur.  The default method simply returns <tt class="xref docutils literal"><span class="pre">True</span></tt>, indicating that by
default, all channels will be interested in write events.</dd></dl>

<p>In addition, each channel delegates or extends many of the socket methods.
Most of these are nearly identical to their socket partners.</p>
<dl class="method">
<dt id="asyncore.dispatcher.create_socket">
<tt class="descname">create_socket</tt><big>(</big><em>family</em>, <em>type</em><big>)</big><a class="headerlink" href="#asyncore.dispatcher.create_socket" title="Permalink to this definition">¶</a></dt>
<dd>This is identical to the creation of a normal socket, and will use the
same options for creation.  Refer to the <a title="Low-level networking interface." class="reference external" href="socket.html#module-socket"><tt class="xref docutils literal"><span class="pre">socket</span></tt></a> documentation for
information on creating sockets.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.connect">
<tt class="descname">connect</tt><big>(</big><em>address</em><big>)</big><a class="headerlink" href="#asyncore.dispatcher.connect" title="Permalink to this definition">¶</a></dt>
<dd>As with the normal socket object, <em>address</em> is a tuple with the first
element the host to connect to, and the second the port number.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.send">
<tt class="descname">send</tt><big>(</big><em>data</em><big>)</big><a class="headerlink" href="#asyncore.dispatcher.send" title="Permalink to this definition">¶</a></dt>
<dd>Send <em>data</em> to the remote end-point of the socket.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.recv">
<tt class="descname">recv</tt><big>(</big><em>buffer_size</em><big>)</big><a class="headerlink" href="#asyncore.dispatcher.recv" title="Permalink to this definition">¶</a></dt>
<dd>Read at most <em>buffer_size</em> bytes from the socket&#8217;s remote end-point.  An
empty string implies that the channel has been closed from the other end.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.listen">
<tt class="descname">listen</tt><big>(</big><em>backlog</em><big>)</big><a class="headerlink" href="#asyncore.dispatcher.listen" title="Permalink to this definition">¶</a></dt>
<dd>Listen for connections made to the socket.  The <em>backlog</em> argument
specifies the maximum number of queued connections and should be at least
1; the maximum value is system-dependent (usually 5).</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.bind">
<tt class="descname">bind</tt><big>(</big><em>address</em><big>)</big><a class="headerlink" href="#asyncore.dispatcher.bind" title="Permalink to this definition">¶</a></dt>
<dd>Bind the socket to <em>address</em>.  The socket must not already be bound.  (The
format of <em>address</em> depends on the address family &#8212; see above.)  To mark
the socket as re-usable (setting the <tt class="xref docutils literal"><span class="pre">SO_REUSEADDR</span></tt> option), call
the <a title="asyncore.dispatcher" class="reference internal" href="#asyncore.dispatcher"><tt class="xref docutils literal"><span class="pre">dispatcher</span></tt></a> object&#8217;s <tt class="xref docutils literal"><span class="pre">set_reuse_addr()</span></tt> method.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.accept">
<tt class="descname">accept</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.accept" title="Permalink to this definition">¶</a></dt>
<dd>Accept a connection.  The socket must be bound to an address and listening
for connections.  The return value is a pair <tt class="docutils literal"><span class="pre">(conn,</span> <span class="pre">address)</span></tt> where
<em>conn</em> is a <em>new</em> socket object usable to send and receive data on the
connection, and <em>address</em> is the address bound to the socket on the other
end of the connection.</dd></dl>

<dl class="method">
<dt id="asyncore.dispatcher.close">
<tt class="descname">close</tt><big>(</big><big>)</big><a class="headerlink" href="#asyncore.dispatcher.close" title="Permalink to this definition">¶</a></dt>
<dd>Close the socket.  All future operations on the socket object will fail.
The remote end-point will receive no more data (after queued data is
flushed).  Sockets are automatically closed when they are
garbage-collected.</dd></dl>

</dd></dl>

<dl class="class">
<dt id="asyncore.file_dispatcher">
<em class="property">
class </em><tt class="descclassname">asyncore.</tt><tt class="descname">file_dispatcher</tt><a class="headerlink" href="#asyncore.file_dispatcher" title="Permalink to this definition">¶</a></dt>
<dd>A file_dispatcher takes a file descriptor or file object along with an
optional map argument and wraps it for use with the <tt class="xref docutils literal"><span class="pre">poll</span></tt> or
<tt class="xref docutils literal"><span class="pre">loop</span></tt> functions.  If provided a file object or anything with a
<tt class="xref docutils literal"><span class="pre">fileno</span></tt> method, that method will be called and passed to the
<a title="asyncore.file_wrapper" class="reference internal" href="#asyncore.file_wrapper"><tt class="xref docutils literal"><span class="pre">file_wrapper</span></tt></a> constructor.  Availability: UNIX.</dd></dl>

<dl class="class">
<dt id="asyncore.file_wrapper">
<em class="property">
class </em><tt class="descclassname">asyncore.</tt><tt class="descname">file_wrapper</tt><a class="headerlink" href="#asyncore.file_wrapper" title="Permalink to this definition">¶</a></dt>
<dd>A file_wrapper takes an integer file descriptor and calls <a title="os.dup" class="reference external" href="os.html#os.dup"><tt class="xref docutils literal"><span class="pre">os.dup()</span></tt></a> to
duplicate the handle so that the original handle may be closed independently
of the file_wrapper.  This class implements sufficient methods to emulate a
socket for use by the <a title="asyncore.file_dispatcher" class="reference internal" href="#asyncore.file_dispatcher"><tt class="xref docutils literal"><span class="pre">file_dispatcher</span></tt></a> class.  Availability: UNIX.</dd></dl>

<div class="section" id="asyncore-example-basic-http-client">
<span id="asyncore-example"></span><h2>18.6.1. asyncore Example basic HTTP client<a class="headerlink" href="#asyncore-example-basic-http-client" title="Permalink to this headline">¶</a></h2>
<p>Here is a very basic HTTP client that uses the <a title="asyncore.dispatcher" class="reference internal" href="#asyncore.dispatcher"><tt class="xref docutils literal"><span class="pre">dispatcher</span></tt></a> class to
implement its socket handling:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">asyncore</span><span class="o">,</span> <span class="nn">socket</span>

<span class="k">class</span> <span class="nc">http_client</span><span class="p">(</span><span class="n">asyncore</span><span class="o">.</span><span class="n">dispatcher</span><span class="p">):</span>

    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">host</span><span class="p">,</span> <span class="n">path</span><span class="p">):</span>
        <span class="n">asyncore</span><span class="o">.</span><span class="n">dispatcher</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">create_socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span> <span class="p">(</span><span class="n">host</span><span class="p">,</span> <span class="mf">80</span><span class="p">)</span> <span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">buffer</span> <span class="o">=</span> <span class="s">&#39;GET </span><span class="si">%s</span><span class="s"> HTTP/1.0</span><span class="se">\r\n\r\n</span><span class="s">&#39;</span> <span class="o">%</span> <span class="n">path</span>

    <span class="k">def</span> <span class="nf">handle_connect</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">pass</span>

    <span class="k">def</span> <span class="nf">handle_close</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

    <span class="k">def</span> <span class="nf">handle_read</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">print</span> <span class="bp">self</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mf">8192</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">writable</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">buffer</span><span class="p">)</span> <span class="o">&gt;</span> <span class="mf">0</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">handle_write</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="n">sent</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">buffer</span><span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">buffer</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">buffer</span><span class="p">[</span><span class="n">sent</span><span class="p">:]</span>

<span class="n">c</span> <span class="o">=</span> <span class="n">http_client</span><span class="p">(</span><span class="s">&#39;www.python.org&#39;</span><span class="p">,</span> <span class="s">&#39;/&#39;</span><span class="p">)</span>

<span class="n">asyncore</span><span class="o">.</span><span class="n">loop</span><span class="p">()</span>
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